Recent Results of SpinDependent Tunneling Planar Tunnel Junctions

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Recent Results of Spin-Dependent TunnelingPlanar
Tunnel Junctions Magnetic STM

• Jürgen Henk
• Max-Planck-Institut für Mikrostrukturphysik
• Halle a.d. Saale, Germany

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Introduction

• Aim Electron spin electronics spintronics
• Devices Spin valves, magnetic tunnel
  transistors, MRAMs,

Theoretical description recent results Planar
tunnel junctions, magnetic STM
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Outline

• Planar tunnel junctions
• Theoretical approaches
• Computational
• Examples
• Spin valve Co/Cu/Co
• Hot spots Ni/Vac/Ni
• Bias voltage Co/Vac/Co
• Interface structure Fe/MgO/Fe
• Magnetic STM
• Scattering-theoretical approach
• Implementation in multiple-scattering theory
• Recent results for a model system
• Details Talk poster by Piotr Karas
• Outlook

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Planar Tunnel Junctions

• Device

Ferromagnetic leads L R
Insulating spacer S
Parallel alignment (P)
Antiparallel alignment (AP)
Measured Tunneling current I, conductance
G Tunneling magneto-resistance (TMR)
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Planar Tunnel Junctions II

• Tunneling magneto-resistance depends on
• Leads Electronic magnetic properties
• Spacer Thickness, electronic magnetic
  properties
• Interfaces Geometry, electronic structure
  magnetic structure
• Bias voltage
• All to be considered by theory

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Theoretical Approaches

• Jullieres model
• Optimistic TMR
• Spin polarization in the leads L R
• Spacer properties completely ignored

Slonczewskis extension

• Spacer step barrier
• Transmission polarization

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Theoretical Approaches II

• Landauer-Büttiker approach
• Leads Homogenous Green function G
• Spacer Transition operator T
• Conductance G

Scattering channels Bloch states
Scattering at the spacer
Sum over all scattering channels paths ? TMR
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Theoretical Approaches III

• Landauer-Büttiker approach formulated in
  layer-KKR (MacLaren Butler)
• Principal building blocks layer
• Typical LEED algorithms
• Ballistic tunneling no disorder
• Transmission
• Conductance

Spacer scattering matrix
Scattering channels Bloch states
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Computational

• First-principles calculations
• Scalar-relativistic spin-polarized KKR
• Self-consistent electronic structure
• Tunneling calculations
• Relativistic spin-polarized layer-KKR
• Landauer-Büttiker approach
• MacLaren-Butler algorithm
• Brillouin-zone sampling by adaptive mesh
  refinement

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Spin Valve Co/Cu/Co

• fcc-Co(001)/Cu/fcc-Co(001)
• Conducting spacer TMR almost constant
• Quantum-well states in the Cu spacer Oscillations

Conductance vs spacer thickness
Hot Spots Ni/Vac/Ni

• Ni(001)/vacuum/Ni(001)
• Insulating spacer Focusing of the transmission
  exponentially decreasing conductance
• Interface resonances Hot spots in the
  transmission

Transmission
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Bias-Voltage Dependence Co/Vac/Co

• Typical observation for oxide barriers Decrease
  of the TMR with bias voltage
• Improved sample preparation lead to less decrease
• Origin Defect scattering, magnons, ?
• Idea Replace the oxide by vacuum ? no defects
• Replace the planar tunnel junction by a STM
  set-up
• Is the TMR still decreasing?

H.F. Ding, W. Wulfhekel, JH, P. Bruno J.
Kirschner, submitted to PRL
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Bias-Voltage Dependence Co/Vac/Co II
Co(0001)/vacuum/Co-STM tip Experimental results
with a magnetic STM
Large tip-sample distance (7 Å)
Small tip-sample distance (5 Å)
TMR constant
Dip at 0.2 eV
TMR vs bias voltage
Experiments by Hai Feng Ding Wulf Wulfhekel
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Bias-Voltage Dependence Co/Vac/Co III

• Theory for a Co(0001)/vacuum/Co(0001) planar
  junction

• Problem Bias voltage
• Simple solution
• Change the inner potentials of the leads (bias)
• Adapt a smooth interface barrier from LEED
• Barrier height automatically adjusted

Barrier height vs lead separation
Interface barrier
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Bias-Voltage Dependence Co/Vac/Co IV
Surface state
Spin-resolved bulk bands

• TMR vs bias voltage

TMR constant
Spin-resolved spectral density of Co(0001)
Conductance vs bias voltage
Theory confirms experiment for large distances
Dip? Not found in theory but possibly related to
the majority surface state. Tip-induced feature?
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Interface Structure Fe/MgO/Fe

• Fe(001)/MgO/Fe(001)
• Bulk electronic structure

MgO
Band gap too small (DFT) 4.24 eV vs 7.6 eV
(Expt.)
Fe
Majority
Minority
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Interface Structure Fe/MgO/Fe II

• Geometry

Bulk Fe bcc
a 2.81 Å
a 2.81 Å
Bulk MgO fcc, NaCl
a 2.87 Å
a 4.05 Å
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Interface Structure Fe/MgO/Fe III

• Interface geometry
• Standard model ideal cut paste structure
• New Formation of an FeO interface layer

FeO interface layer
O
X-ray diffraction Holger Meyerheim et al, PRL 87
(2001) 076102 Confirmed by total-energy
calculations (Arthur Ernst) Gain of 0.73 eV/atom
Empty spheres
Mg
Effect of the interface structure on the TMR?
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Interface Structure Fe/MgO/Fe IV

• Charge redistribution magnetic profiles for 2
  MgO layers

Magnetic moments
Charge distribution
Standard model (Butler et al.)
Spin-down depletion
Fe interface layer with empty spheres
New model FeO interface layer
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Interface Structure Fe/MgO/Fe V

• Transmission for 2 MgO layers

Fe interface layer with empty spheres
FeO interface layer
P alignment
AP alignment
Fe/(MgO)4/Fe
Fe/FeO/(MgO)2/FeO/Fe
Additional scattering at the FeO layer ? Change
of the transmission
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Interface Structure Fe/MgO/Fe VI

• Conductance TMR

Pre-asymptotic regime increasing TMR
Strong influence of the interface for thin
spacers (2-4 ML MgO)
Exponential decay
Bulk potentials only
Fe interface layer with empty spheres
FeO interface layer
Preliminary results Work in progress
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STM Multiple-Scattering Approach

• Motivation Beyond the Tersoff-Hamann model
• Electronic structure of tip sample
• Tip-sample interaction
• Magnetic STM
• Light-assisted tunneling,

Tunneling as a scattering process Formulation
implementation in KKR
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STM Multiple-Scattering Approach II
Impermeable membrane

• Theoretical outline
• 3 systems A, B S
• Separation of A B
• Membrane (Pendry)
• Here Spatial temporal translation

Left lead (sample, A)
Right lead (tip, B)
Interacting system (tip sample, S)
Offset vector
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STM Multiple-Scattering Approach III

• Theoretical outline II
• Spatial temporal translation

Translation
Change of the potential at the interface
Interaction between A B
Sudden approximation All systems in their ground
state ? DFT can be used.
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STM Multiple-Scattering Approach IV

• Theoretical outline III
• Application of scattering theory
• Hamiltonians of A, B S
• Translation operators for A B
• Green functions
• Transition, Møller wave Lovelace operators
• Elastic tunneling current from A to B
• Total current
• All ingredients available in KKR!
• Implementation in the omni2k program package

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STM Multiple-Scattering Approach V

• Results for a magnetic model system

• Empty spheres Cu(001) structure
• Parameters
• Tip wavefunction Depth of the tip well
• Exchange splitting

Increase with exch. splitting
1 eV exch. splitting
2.5 eV
0.5 eV
Sign change
Spin-split resonance
Spectral density vs tip potential
TMR vs tip potential exchange splitting
For details Poster talk by Piotr Karas
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Outlook Work in Progress

• Fe/MgO/Fe
• Consider the partial occupation in the FeO layer
  (coherent potential approximation)
• Inclusion of electron correlation (oxide layer)
  via self-interaction correction
• Full-potential calculations
• Co/Au/Vac/Co
• Effect of quantum-well states in the Au film
• Magnetic STM
• Application to real systems
• Bias voltage, tip shape, full potential,
• Time-dependent processes

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Drivers, supporters, observers,
Jamal Berakdar, Mohammed Bouhassoune, Patrick
Bruno, Markus Däne, Hai Feng Ding, Arthur Ernst,
Wolfram Hergert, Piotr Karas, Jürgen Kirschner,
Martin Lüders, Udo Schmidt, Dzidka Szotek, Walter
Temmerman, Wulf Wulfhekel,